Project Summary We have recently found that B1 cells make large amounts of IgE post-helminth infection and that the B1 cell IgE acts contrary to B2 antigen (Ag)-specific IgE. B1 cell IgE prevents mast cell degranulation through competition for the Fc?RI. Recently published data also shows that this ?benign? B1 cell IgE blocks B2 cell IgE- mediated egg suppression. This B1 cell IgE production assists the helminth in immune evasion and benefits life and reproduction. Further, we show that the helminth-induced alarmin, IL-25, is responsible for the induction of B1 IgE production in vivo. These findings will be further investigated with the following three aims. In Aim 1, using a newly developed mouse that makes FLAG-tagged secreted IgE, we will determine the relative levels of B1 cell vs B2 cell IgE seen post helminth infection. V(D)J region sequencing will be examined for B1 cells and B2 cells before and after helminth infection. B1 cell IgE will be examined for its protective effects after helminth infection in allergic airway models. Aim 2 will examine the mechanism of IL-25 B1 IgE enhancement. Using the FLAG-IgE system the levels of IgE made by IL-25R-/- B1 cells when reconstituting with WT B2 cells will be compared. IL-25R expression analysis on the B1 cell compartments will be assessed. IL-25 signaling in B1 cells will be examined, particularly with respect to STAT5 and STAT3 induction. Aim 3 will move into the human system where preliminary data shows IL-25R on a portion of B cells that produce IgE in response to IL-25 isolated from tonsil. The surface phenotype of the IL25R+ B cell will be more completely characterized and compared to published ?B1-like? human B cell markers. PBMC and cord blood B1 cells will be assessed for IL-25/IL-25R signaling. We will additionally examine B cells in PBMC from allergic vs non- allergic individuals to determine if there are differences in the levels of the IL25R+ B cells in the two populations, which could indicate a bias towards allergic sensitivity. A better understanding of non-specific IgE induction could ultimately translate into more effective allergic disease treatments.